control and safety sytem for small skids

Sounds like another homework problem to me.

This will depend on exactly what the skid contains and what needs to be controlled. More details are needed in order to suggest an optimal control system, especially if hazardous or highly valuable fluids are involved.

The skid contains water feed pit ,transfer pump to two tanks and two disposal pumps

the fluid is not valuable it is Produced water

of course the control point will be basically the level of the feed pit & tanks ,the discharge pipe lines pressure and flow and the protection of the pumps and motors

regards

Safety (such as E-Stops) should be built into your control program. There is really no better way to do it. Your control program oversees the entire system - it is only logical that all I/O be directed by it, of which the safety system is but a part.

Anything that has a personnel safety function needs to be hard-wired fail-safe, and not go via any software.

For pumps and tanks:

• Consider putting emergency stop buttons and lockable isolators next to each pump.
• Consider the implications of a wire falling off the terminals: does that high level switch break on high level so as to interrupt the supply to the pump?
• Consider hard-wired "vital" interlocks on the pumps: a low-level switch that breaks to interrupt the pump power supply when the tank has nearly been pumped dry irrespective of whatever else is going on in the control software, for example.

SIL certification is of no value without the accompanying test documentation.